When taking a photograph of a subject under backlit conditions, e.g. against the sun, the image of the subject portion is an indistinct backlit image with low lightness and contrast, making it difficult to make out details of the subject. Image processing is performed to improve the quality of these backlit images, as well as images suffering in quality due to overexposure, underexposure, blurring due to jiggling when capturing the image, noise, insufficient light, and the like, in order to improve lightness and contrast in the image. One method used in conventional image processing is a Retinex process.
As disclosed in U.S. Patent Application Publication No. 2004/0091164 (corresponding to Japanese Patent No. 3,731,577), the Retinex process is performed to correct lightness and contrast, such as gamma correction of illumination components. The Retinex process can produce an output image with improved image quality for low-quality regions of the input image, such as backlit regions.